As described in copending patent application Ser. No. 742,280 filed June 7, 1986 (now U.S. Pat. No. 4,657,472) a standard manipulator has a stationary base on which a carousel can be rotated about a vertical axis by a motor mounted on the base. A main support arm is pivotal on the carousel about an inner horizontal axis perpendicularly intersecting the vertical axis by means of another motor mounted on the carousel. A counterweight is provided for counterbalancing the offcenter weight of the main arm and the structure carried by it.
The outer or upper end of the main arm carries an outrigger arm rotatable on the main arm about an outer horizontal axis by means of yet another motor carried on the main arm. The outer end of this outrigger arm in turn carries a so-called head assembly that in turn carries the tool, for instance a welding electrode.
This head assembly itself comprises inner, intermediate, and front segments pivotal about respective axes, with the outer and inner axes forming with the intermediate axis respective outwardly and inwardly open acute angles.
The inner side of the rear segment carries the drive shafts for the head parts and also for a tool holder concentric to the front segment. Drives with high stepdowns, at least 50:1, connect the shafts to the respective parts while bevel gearing connects succeeding shafts to one another.
This arrangement is supposed to provide the maximum reach for the tool while being as compact as possible. As a result of the relative inclinations of the various axes it is possible to direct the tool from the front segment inward back toward the support formed normally by the outer end of the outer arm. In fact the tool can lie on an axis parallel to the base axis, which typically is the longitudinal axis of the outer arm, and can point from the outer head part back to the base.
In such devices there are three concentric drive shafts in the front segment. Since normally there are four different relatively movable parts (for instance the three head segments and the tool) it is not uncommon to couple two of the segments together so they can both be driven by one of the shafts.
As also described in German patent document No. 3,431,033 the inner and front segments are coupled together by a bevel gearing. In this manner the front segment rolls off on the rear segment as the intermediate segment is rotated.
In such systems the shafts can be fairly slim and compact since the high stepdown at the outermost point in the gear train makes any minor losses or inefficiencies that exist upstream in the drive train inconsequential. Slip of, for instance, 1.degree. in an upstream gear train is reduced by a standard harmonic transmission with a 100:1 ratio to 0.01.degree., a normally negligible amount.
The transmissions are of the high-ratio and extremely compact harmonic type which basically have three parts. The rear segment or so-called wave generator is of elliptical section and supports via an array of rollers a flexible toothed belt or so-called flexspline having external teeth that mesh at two diametrically opposite locations with two juxtaposed ring gears.
One of these ring gears is called the circular spline, is normally fixed, and normally has two more teeth than the flexspline. The other ring gear is called the dynamic spline, is normally rotatable, but has the same number of teeth as the flexspline.
Such a transmission is extremely compact and is capable of transmitting considerable torque. In addition it has very little slip, and converts torque purely into torque so that it does not load the support bearings radially or axially. The ratio in such a transmission is normally at least 80:1.
Such transmissions are braced, therefore, between relatively moving parts so that calculating the position of the tool held on the outer head segment becomes a fairly complex task.
In addition such setups normally preclude providing a separate rotary drive for the tool holder on the outer face of the front segment.